Professor Anastasia V. Bochenkova

Department of Chemistry, Lomonosov Moscow State University, Russia

Light of various wavelengths and intensity can be used as a probe of molecular structure and dynamics. Absorption and photoelectron spectroscopies provide invaluable information not only about the electronic structure of molecules, but also about the photoinduced dynamics of vibrational wave packets in their excited states. Knowledge of vibrational modes, which become active upon electronic transitions, can greatly enhance our understanding of mechanisms, specificity, and speed of primary photochemical reactions, such as photoisomerization in human vision and photoinduced electron transfer in photoactive proteins. Vibronic couplings are essential for transitions otherwise electronically forbidden by symmetry; however, even for allowed transitions, vibronically induced contributions can be significant and define, for example, the origin of markedly different spectral shapes in one- and two-photon absorption. We will discuss mechanisms of radiative and non-radiative transitions and approaches for modeling vibronic band shapes of polyatomic molecules using a linear coupling scheme, which accounts for both the Franck-Condon and Herzberg-Teller contributions. We will further analyze the differences in one-photon and two-photon absorption profiles for the lowest-lying transitions using a simple two-level model, which allows one to describe two-photon cross-sections and vibronic contributions using readily available physical properties of the two states involved in the transition. We will then apply the basic linear model for simulating and analyzing one-photon and two-photon spectral profiles of retinal-containing visual and microbial rhodopsins and the green fluorescent protein (GFP). We will show that the photoresponse of the retinal chromophore is highly mode-specific inside the proteins, resulting in excitation of those vibrational modes that facilitate photoisomerization of a particular double bond. Furthermore, by calculating instantaneous population of vibrational modes, which mediate barrier-controlled photochemical reactions in excited states, such as nuclear-driven photoinduced electron transfer in GFP, we can also estimate their non-thermal rate constants as a function of excitation wavelength in the linear and non-linear regime. Finally, we will extend our discussion to simulations of non-resonant and resonant photoelectron spectra of isolated biologically relevant chromophore anions to identify direct and indirect electron emission processes and to reveal molecular resonances and weakly bound electronically excited states as gateways for electron transfer processes. Non-adiabatic coupling matrix elements in the nuclear configuration space will be used to redefine Franck-Condon integrals for treating vibrational autodetachment of photoelectrons.

References:

P.A. Kusochek, A.V. Scherbinin, A.V. Bochenkova. Insights into the Early-Time Excited-State Dynamics of Structurally Inhomogeneous Rhodopsin KR2. J. Phys. Chem. Lett. 2021, 12, 8664–8671, DOI: 10.1021/acs.jpclett.1c02312.

C.S. Anstoter, G. Mensa-Bonsu, P. Nag, M. Rankovic, K. Ragesh T. P., A.N. Boichenko, A.V. Bochenkova, J. Fedor, J.R.R. Verlet. Mode-Specific Vibrational Autodetachment Following Excitation of Electronic Resonances by Electrons and Photons. Phys. Rev. Lett. 2020, 124, 203401. DOI: 10.1103/PhysRevLett.124.203401.

A.V. Bochenkova, C.R.S. Mooney, M.A. Parkes, J.L. Woodhouse, L. Zhang, R. Lewin, J.M. Ward, H.C. Hailes, L.H. Andersen, H.H. Fielding. Mechanism of resonant electron emission from the deprotonated GFP chromophore and its biomimetics. Chemical Science 2017, 8, 3154-3163. DOI: 10.1039/C6SC05529J.

Professor Samer Gozem

Department of Chemistry, Georgia State University, Atlanta, GA 30303, USA

UV-visible and photoelectron spectroscopy are powerful tools for probing the structure of
matter from the subatomic to the bulk scale. The experimental spectra are generally
plotted using two properties: energies and absorption strength (the latter typically reported
as molar attenuation coefficients or cross sections). Energies and transition strengths
could also be predicted from first principles with quantum chemical methods. In the gas
phase, experiments and computations can be reconciled when the appropriate quantum
chemical methods are used. In the condensed phase, however, experimental spectra are
shifted and broadened by intermolecular interactions that complicate the comparison
between theory and computations. At the same time, the condensed-phase spectra
encode potential important information about these intermolecular interactions and how
they modulate a solute’s electronic structure. The first part of the presentation will cover
the basics of computational spectroscopy, and discuss how computed energies and
intensities can be compared with experimental ones. The second part of the presentation
will bring the computations into the condensed phase with hybrid quantum chemical /
molecular mechanical (QM/MM) models, which can be used to understand the effect of a
solvent (or a protein host) on the spectroscopic properties of a solute (or cofactor).

References:

1. Gozem, S.; Krylov, A.I. The ezSpectra suite: An easy‐to‐use toolkit for
spectroscopy modeling. WIREs Comp. Mol. Sci. e1546. 2021.
2. Tarleton, A.; Garcia-Alvarez, J.; Wynn, A.; Awbrey, C.; Roberts, T.; Gozem, S.
OS100: A Benchmark Set of 100 Digitized UV-Visible Spectra and Derived
Experimental Oscillator Strengths. ChemRxiv 2021. This content is a preprint and
has not been peer-reviewed.
3. Dratch, B.D.; Orozco-Gonzalez, Y.; Gadda, G.; Gozem, S. The Ionic Atmosphere
Effect on the Absorption Spectrum of a Flavoprotein: A Reminder to Consider
Solution Ions. J. Phys. Chem. Lett. 12 (34), 8384–8396. 2021.
4. Orozco-Gonzalez, Y.; Kabir, M.P.; Gozem, S. Electrostatic Spectral Tuning Maps
for Biological Chromophores. J. Phys. Chem. B. 148, 4813—4824. 2019.